vm-migration: Use vm-memory's ByteValued instead of own AsBytes

This addresses a newly added clippy issue:

error: methods called `as_*` usually take self by reference or self by mutable reference; consider choosing a less ambiguous name
  --> vm-migration/src/protocol.rs:57:34
   |
57 |     fn as_mut_bytes<T: Sized>(p: &mut T) -> &mut [u8] {
   |                                  ^^^^^^
   |
   = help: for further information visit https://rust-lang.github.io/rust-clippy/master/index.html#wrong_self_convention

error: aborting due to 2 previous errors

error: could not compile `vm-migration`

Signed-off-by: Rob Bradford <robert.bradford@intel.com>

vm-migration/Cargo.toml

@@ -10,3 +10,4 @@ thiserror = "1.0"
 serde = {version = ">=1.0.27", features = ["rc"] }
 serde_derive = ">=1.0.27"
 serde_json = ">=1.0.9"
+vm-memory = { version = "0.5.0", features = ["backend-mmap", "backend-atomic"] }